Analysis and Spectra Flashcards
Molecular spectra, whether NMR, IR, or mass spec, can be intimidating when they appear on the MCAT. Use these cards to master equivalent protons, memorize important shifts, and learn how these complex methods relate.
What do infrared (IR) and ultraviolet (UV) spectroscopy have in common?
Both techniques involve absorption of electromagnetic radiation by the molecule that is being analyzed.
Radiation with a specific frequency will be absorbed by the electrons in a molecule, exciting them to higher energy states.
What happens if a molecule is simultaneously exposed to several frequencies of light?
The molecule will only absorb radiation from the particular frequencies at which it has matching energy states. This concept is known as resonance.
Radiation with other frequencies will pass through or reflect. Spectroscopy, or the process of measuring the absorbed radiation, can yield information about the composition of the molecule.
What can infrared (IR) spectroscopy identify about an organic molecule?
functional groups present on the molecule
Different covalent bonds vibrate at different, characteristic frequencies when light is absorbed by the molecule. These frequencies correspond to certain common groups.
In infrared (IR) spectroscopy, how can molecules with exactly the same atoms absorb light differently?
Different covalent bonds vibrate at different, characteristic frequencies. These vibrations can exist in different forms, such as stretching, bending, and rocking.
Molecules that contain the same atoms but different functional groups will absorb light at their specific characteristic frequencies. Their spectra will not necessarily overlap.
What determines the frequency of an IR vibration?
- number of electron pairs
- polarity of the bond
- masses of the atoms involved
What qualities characterize bonds with high IR vibration frequencies?
- double and triple bonds
- polar bonds
- bonds that include lighter atoms
Remember, IR vibration frequencies are determined by the characteristics of the bond.
What qualities characterize bonds with low IR vibration frequencies?
- single bonds
- nonpolar bonds
- bonds that include heavier atoms
Remember, IR vibration frequencies are determined by the characteristics of the bond.
How can a molecule be identified using an IR spectrum?
It is identified via characteristic peaks in its spectrum.
Though functional groups may have many absorption peaks across the full spectrum, certain ranges are important to know.
Which functional group yields the IR peak shown below?
hydroxyl (-OH)
Hydroxyl groups always give a broad absorption peak in the range of 3100-3500 cm-1.
Which functional group yields the IR peak shown below?
carbonyl (C=O)
Carbonyl groups always give a sharp absorption peak around 1700 cm-1.
Which functional group yields the IR peak shown below?
amine (N-H)
Amine groups always give a sharp absorption peak in the range of 3100-3500 cm-1.
What functional group must be present in a molecule with the IR spectrum shown below?
carbonyl
Carbonyl groups always give a sharp absorption peak around 1700 cm-1. The graph above is the spectrum of propanone.
What functional group must be present in a molecule with the IR spectrum shown below?
hydroxyl
Hydroxyl groups always give a broad absorption peak in the range of 3100-3500 cm-1. The graph above is the spectrum of propan-1-ol.
What functional group must be present in a molecule with the IR spectrum shown below?
amine
Amine groups always give a sharp absorption peak in the range of 3100-3500 cm-1. The graph above is the spectrum of 1-aminobutane.
What functional group must be present in a molecule with the IR spectrum shown below?
hydroxyl
Hydroxyl groups always give a broad absorption peak in the range of 3100-3500 cm-1. The graph above is the spectrum of ethanol.
What is the fingerprint region of an IR spectrum?
The area of the spectrum between 500 and 1450 cm-1.
Every molecule’s fingerprint region is unique, but the signal in this area tends to be complex. For the MCAT, don’t worry about identifying peaks in this area.
What region of the IR spectroscopy spectrum can generally be disregarded?
The fingerprint region, which covers the range from 500 cm-1 to 1450 cm-1.
The fingerprint region often does not contain distinct characteristic peaks.
What can UV-visible spectroscopy identify about an organic molecule?
electronic energy levels
Electrons bound to different molecules will have different excited states. These molecules will absorb photons of light that correspond to the difference between the ground and excited energy levels, which exist in the visible and near-UV regions of the spectrum.
What features characterize the organic molecules that display signals in UV-visible spectroscopy?
It must have nonbonding electrons that can be excited without breaking a bond.
The most common examples are:
- conjugated molecules (most important for the MCAT)
- molecules with double and triple bonds
- molecules with nonbonding pi electrons
- molecules that contain transition metals
What does the color of an organic molecule indicate about the frequency of light that it absorbs?
Molecules are complementary in color to the light that they absorb.
For example, if a molecule is illuminated with white light and absorbs green light, it must reflect light from the remainder of the spectrum, namely red. The molecule will appear reddish in color.
What color of light must be absorbed by a molecule that appears reddish in color?
green light
More broadly, the absorbed light must have a frequency around the yellow/green/blue area of the spectrum.
A molecule that absorbs green light will reflect red, making it appear reddish in color.
What color of light must be absorbed by a molecule that appears greenish in color?
red light
A molecule that absorbs red light will reflect light in the blue/green/yellow area of the spectrum, causing it to appear green in color.
What does a mass spectrometer measure?
It measures the path curvature of molecular fragments from a sample. This is then related to a “mass-to-charge ratio” for each fragment.
The mass-to-charge ratio can then be used to calculate molecular mass.
Briefly describe how a mass spectrometer works.
It bombards a vapor sample with sufficiently high-energy electrons to both fragment and ionize the sample.
The charged ions and fragments are then accelerated using an electric field and bent through a curved detector using a magnetic field. The radius of curvature for the ions depends on their mass and charge, allowing total mass to be calculated.